1. FIELD OF THE INVENTION: This invention relates to defect inspection of tubular elements comprising a generally continuous tubular string used in a subterranean oil and gas well, and more particularly to inspection of tubular elements by detecting the properties of magnetic fields induced in the tubular elements during removal from the well.
2. DESCRIPTION OF THE PRIOR ART: Continuous tubular strings formed of connectable tubular sections or elements, such as production tubing strings, strings of drill pipe and casing strings, are used in the drilling, completion and production of subterranean oil and gas wells. The tubular elements comprising such strings are subject to mechanical damage while the tubular elements are located within the well and are also subject to the action of corrosive fluids which may be contained within the tubular elements or which may be transported through the tubular string between the well surface and a downhole location. It is therefore advantageous that the individual tubular elements comprising a tubular string be inspected periodically. Commonly, tubular elements or tubular sections are inspected for defects after the tubing string is removed from the well. Conventional inspection of tubular sections normally occurs after the individual tubing sections comprising the tubing string have been disengaged. Defect inspections are conventionally performed on a section by section basis. Occasionally, inspection is performed downhole through the use of inspection calipers. These tools leave "caliper tracks" and can be affected by the contents of the tubing.
A number of techniques exist for determining the presence of a defect in a tubing section. For example, the precise location of internal and external radially extending and three dimensional defects, including slug inclusions, mechanical damage, corrosion pitting and fatique cracks has been determined by flux leakage techniques in which a longitudinal magnetic field is induced by one or more magnetic induction coils. Surface riding detectors are located around the tubing and the maximum signal is recorded to precisely locate the defect. Since this magnetic inspection is conducted on a section by section basis after disengagement from the tubing string, when surface dirt, scale and mud can be controlled, detectors can be placed directly on the surface of the ferromagnetic tubular section to determine the presence of defects.
The location of longitudinal defects, including internal and external seams, plug scores, eccentricity, wear due to sucker rod interference, and wireline cuts, has been detected by inducing a circumferential magnetic field in the tubing. The field is induced by a high current discharge through an insulated rod on the interior of the tubing section. Detectors rotating around the surface of the tubing locate these longitudinal defects. Again, since the inspection is conducted on a section by section basis, the insulated rod can be inserted through the interior of the tubing section for this longitudinal defect inspection technique.
Other conventional inspection systems use methods which do not require insertion of rods and probes inside the pipe. Specifically, a common way of detecting longitudinal defects magnetically is the "rotating pole" method, where the magnetic field is applied from the outside by rotating electromagnets, and detectors positioned in between the poles scan the outside surface of the pipe.
Tubing wall thickness has been measured by measuring the radiation from a rotating radioactive source of gamma radiation transmitted through the wall of a tubing section. For example, with a source rotating around the pipe a detector may be located on the inside of the pipe to determine the degree of attenuation of gamma radiation and thus to determine the wall thickness. Again, this technique requires access to individual tubing sections after disengagement of the string. Other ways of measuring wall thickness with gamma radiation, which are backscatter, double-wall through-transmission and chord, have both the radiation detector and the source located on the outside of the pipe. Radiation methods introduce such problems as radiation licensing, record keeping, radiation safety administration, source handling complications, and fear of the unknown.
Techniques requiring surface-riding detectors, insertion of a detector or a driving means within the bore of tubular elements or requiring rotating mechanical means to obtain a complete circumferential coverage of tubing sections are unsuited for use is defect inspection and measurement of tubing sections while the string is being removed from the well. These defect inspection techniques are also unsuited to the measurement of defects in tubing sections while the sections are interconnected in the tubing string. Thus these inspection techniques are not suitable for use on a drilling, completion or workover rig at the surface of the well to measure defects in a tubing string as the string is removed from the well. In addition to the requirements that only disengaged tubing sections be individually measured, additional problems which would be encountered are the limited space available on the rig, the inability to control the longitudinal velocity of the tubing string as it is removed from the well, and the difficulty in precisely controlling the transverse location of the tubing sections comprising the tubing string. Furthermore, the use of surface detectors in a tubing trip tool for measuring defects as tubing sections, comprising a tubing string, are removed from the well is also complicated by the presence of solid deposits, such as drilling mud, and tubing mounted components, such as retrievable packers, which may be incorporated into the tubing string. One technique for inspecting tubular elements which is adaptable to relative movement, at variable velocities, is a technique involving the use of a saturating longitudinal magnetic field and the subsequent measurement of the time integral of the electrical signal caused by the magnetic field applied to the ferromagnetic tubular member to determine the average wall thickness. Testing using this technique has been conducted for surface pipe installations in which the magnetic field and the flux detecting elements are moved relative to a continuous pipe array. Such apparatus has not, however, been employed to measure the average wall thickness of tubing sections as they are removed from an oil or gas well.